Systems for controlling the automatic tracking in high frequency antennas



y 1968 J. RENAUDIE SYSTEMS FOR CONTROLLING THE AUTOMATIC TRACKING INHIGH FREQUENCY ANTENNAS Flled Oct 22 1965 5 Sheets-Sheet 1 FlgflclF|g.2c|

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- May 14, 1968 J. RENAU DIE SYSTEMS FOR CONTROLLING THE AUTOMATICTRACKING IN HIGH FREQUENCY ANTENNAS Filed Oct. 22, 1965 FIG.4

3 Sheets-Sheet 2 I VVENTO May 14, 1968 J. RENAUDIE 3,383,688

SYSTEMS FOR CONTROLLING THE AUTOMATIC TRACKING IN HIGH FREQUENCYANTENNAS 7 Filed Oct. 22, 1965 s Sheets-Sheet a United States PatentABSTRACT OF THE DISCLOSURE Control system for effecting automatictracking of an object emitting a tracking signal by a high frequencyreceiving antenna wherein the modes TE and TE are extracted from acylindrical waveguide connected to the antenna and form the sum anddifference, signals for determining tracking error. The modes arederived from pairs of cavities positioned on the circumference of thecylindrical waveguide at the extremities of angularly disposed diametersthereof.

The present invention 18 directed to improvements relating to automatictracking systems for high frequency antennas and more particularly to asystem serving for the excitation of a servornechanism for the alignmentof a receiving antenna in a satellite tracking system, which system hasan excellent efiiciency together with a great structural simplicity.

In order to receive a signal within the range of high frequenciestransmitted by a satellite, particularly a telecommunications satellite,it is known that the axis of maximum sensitiveness of a receivingantenna, which may be, for example, a conical horn, must be preciselypointed toward the satellite, and this condition is maintained in oneknown manner with movement of the satellite by means of a servomechanismexcited by two input signals which are furnished by the demodulation oftwo signals, one signal commonly referred to as the sum signal and theother signal commonly referred to as the difference signal. It is knownparticularly to effect a first demodulation between a first sum signaland the difference signal which furnishes, for example, a correctingsignal in azimuth, and a second demodulation between a second sumsignal, derived from the first sum signal by squaring, and saiddifference signal which provides a signal of correction in elevation.Advantageously, the sum signal does not depend upon the accuracy ofpointing of the directional axis of the antenna toward the satellite. Onthe other hand, the difference signal depends heavily upon the accuratepointing thereof and is therefore useful in conjunction with the sumsignal to 4 determine a tracking error signal.

The geometry of the radiation diagram between a satellite and a conicalhorn receiving antenna has been described in detail in the article by J.S. Cook and R. Lowell, entitled The Auto Track System, published in TheBell System Technical Journal, vol. 42, July 1963, No. 4, Part 2, pagesl2831307. The referenced publication discusses the tracking problemsrelating to the misalignment of the conical horn receiving antenna withthe line of sight to the satellite and the generation of correctingsignals for adjusting the position of the receiving antenna in responseto the detected modes of the energy received by said antenna. Ingenerating error signals for correcting antenna tracking positionadvantage is made of the information which can be derived from theenergy modes of signals received from the satellite. For this purpose,the TE mode in the cylindrical waveguide con- "ice nccted to the outputof the conical horn, referred to as a sum signal provides a referenceagainst which the phase and amplitude, as determined by the TM mode, canbe measured.

The described system includes a receiving antenna, equipped withautomatic tracking in the form of a conical horn terminating in acylindrical waveguide and is provided with an arrangement of cavitiesplaced on the outer surface of the cylindrical guide and excited throughslots in the waveguide by the waves propagated therein, the cylindricalwaveguide, in turn, exciting a pair of rectangular guides. Thisarrangement furnishes two partial sum signals originating from theexcitation of the cylindrical T E mode in two respective orthogonaldirections and an error signal originating from the excitation of thecylindrical mode TM It is known in accordance with this prior art systemthat the TM mode, referred to hereinafter as the difference signal hasan amplitude which is proportional to the pointing error 0 of theantenna and a phase proportional to 5, the direction of the pointingerror. On the other hand, the TE mode or sum signal can be utilized as areference because the magnitude thereof is invariant with the pointingerror 0, when 0 is small, and the mode is excited such that thepolarization of the incoming signal is also preserved regardless of thepointing error, when 0 is small. Thus, the measurement of any twoorthogonal components of the mode TE will determine the polarization ofthe transmitted signal. The mode can therefore be utilized effectivelyas a phase and an amplitude reference.

In the known system, the horizontal and vertical components of the TEmode are sampled separately and constitute the two sum signals whichwith the difference signal derived from the TM mode serves to define thetracking error of the antenna. However, this system operates effectivelyonly if the polarization of the transmitted signal is circular, or atworst only slightly elliptical, since for circular polarization the TMmode will have a null point on the antenna axis, which characteristicsforms the basis for the servocontrol of the antenna in accordance withthe known system.

On the other hand, if the polarization of the transmitted signal islinear, instead of circular or elliptical, the TM mode, characterized bya radial electric field vector, will be insensitive to errors orthogonalto the plane of polarization. If on a portion of the trajectory thereof,the satellite emitting a linearly polarized signal preserves a directionof constant radial displacement with an orientation of its emittingantenna such that the wave emitted is in quadrature with the directionof displacement of the satellite, and thus also in quadrature with theelectric field of said mode, an error signal will not be receiveddespite the pointing error of the antenna.

When the polarization of the transmitted signal is linear, an errororthogonal to the plane of polarization will not excite radiallysymmetric electric fields in the antenna, and thus the beneficialcharacteristics of the T M mode are lost with such an arrangement. Thebasic cause of this problem is that for a circularly polarized signalthe TM mode experiences a null point on the antenna axis; whereas, for alinearly polarized signal, the mode experiences a null plane orthogonalto the axis of polarization.

The problems relating to the attempt to control tracking of a satellitetransmitting linearly polarized signals using the TM mode are wellrecognized. Indeed, it is well known that under these circumstances thetarget will eventually slip away in the null plane of the system. Yet,one of the only suggested solutions to date is to introduce a smallcross polarization component into the transmitted signal to restore thenull point; however, this compromise reduces the response time andotherwise eliminates or assasss 3 reduces the advantages attendant tothe use of linear polarized transmission.

In order to obviate this difficulty, the present invention has theobject of furnishing a non-zero error signal for a received wave havingany direction of polarization. For obvious reasons, the realization ofthis object necessitates the use of a mode wherein the electric fieldvector does not have a symmetry of revolution, this mode being utilizedto provide a pair of difference signals along two different directionsin the guide.

It is an object of the present invention to provide an automatictracking control system for high frequency antennas which avoids all ofthe difficulties and disadvantages inherent in prior art devices.

It is another object of the present invention to provide an automatictracking control system for high frequency antennas which furnishes apositive error tracking signal regardless of the polarization ororientation of the received tracking signal.

it is a further object of the present invention to provide an automatictracking control system for high frequency antennas which providesaccurate tracking under all conditions without material increase inequipment over previous devices of less dependabiilty.

In accordance with the present invention, there is provided a system ofservocontrol for a tracking antenna operating from a pair of signalscalled a sum signal and a difference signal, wherein the sur signal isobtained from a mode TE propagated in a cylindrical guide and extractedfrom two different diameters within the guide forming therebetween anangle a, by means of two pairs of cavities placed on the circumferenceof said cylindrical guide at the extremities of the two diameters, andthe difference signal is obtained from a mode TE propagated in saidcylindrical guide and extracted along two different diameters of theguide forming therebetween an angle b by means of two pairs of cavitiesplaced on the circumference of the guide at the extremities of the twodiameters of the angle b.

These and other objects, features and advantages of the invention willbecome more clearly understood when considered in light of the followingspecific description of the invention taken with the accompanyingdrawings, which illustrate one exemplary embodiment of the instantinvention, wherein:

FIGURES 1a and lb show a field pattern of the cylindrical mode TEproducing the pair of sum signals;

FIGURES 2a and 21) show the field pattern of the cylindrical mode TEproducing the pair of difference signals;

FIGURE 3 shows in schematic form a preferred embodiment of the presentinvention;

FIGURE 4 is a schematic diagram of a system, within the framework of thepresent invention, for obtaining the necessary corrective signals forantenna control utilizing the instant invention, and

FIGURES 5a and 5!) illustrate a second embodiment of the presentinvention.

Lookin more particularly to the drawings, FIGURE 1 shows at (a) and (1))two diagrams of the field pattern of the dominant mode T13 excited in acylindrical guide connected to receiving horn taken along two diametricdirections of the guide separated by an angle of 45. FIGURE 2 shows at(a) and (b) two diagrams of the field pattern of the mode TE excited inthe aforementioned guide taken along the same two directions.

FIGURE 3 illustrates schematically a cylindrical guide 1 in which a modeTE represented by the arrows in continuous lines, such as f, is excitedby the received signal according to FIGURE la and a mode TE representedby the arrows in broken lines, such as f, is excited according to FIGURE2a.

These two modes according to this orientation are collected by a singlepair of cavities C and C placed on the circumference of theaforementioned cylindrical guide 1 on respective ends of a firstdiameter 2 and each are provided with an excitation orifice, such as e.The cavities C and C are excited in phase by the mode TE and the samecavities are excited in phase opposition by the mode TE as is indicatedby the arrows in continuous lines, such as g, and the arrows in brokenlines, such as g. The cavities C and C in turn excite respectively tworectangular guides G and G by means of orifices, such as c.

There exists a second device on the guide identical to the preceding onecomprising two cavities C and O; on a respective end of a diameter 3 at45 to the aforementioned diameter 2, exciting two rectangular guides Gand (i from the energy modes TE and TE in a second orientation inconformity with FIGURES lb and 2b. In order not to overly complicateFIGURE 3, the modes excited according to this second orientation havenot been indicated by arrows. In addition, the cavities C and C excitetwo rectangular guides G and G by means of orifices, such as e.

The arrangement of FIGURE 3 which extracts energy of the modes TE and T5from the cylindrical guide connected to the receiving horn antenna alongtwo diametric directions of the guide is utilized in a system forproviding corrective signals for antenna control, such as provided inFIGURE 4. In FIGURE 4 reference numerals 141, C O (l -G refer to theelements designated by corresponding symbols in FIGURE 3.

All of the transmission paths designated G through G and G arerectangular waveguides of the type conventionally used for wavetransmission at high frequencies. The elements T through T are combinedTs of the type well known in the microwave art. The elements M and M aremagic Ts, equally well known, which pro vide difference outputs d and drespectively, and sum outputs s and s respectively. In the path betweenthe sum output s of magic T M and the T T there is provided anattenuator A providing an attenuation of 1/2. In the path between thesum output s of the magic T M and the T T there is provided anattenuator A providing an attenuation of /2/2.

A phase shifter D providing a phase shift of sir/2 is connected in theline between the difference output d of magic T M and the T T A phaseshifter D providing a phase shift of 1r is provided between Ts T and Tand a phase shifter D providing a phase shift of eir/Z is providedbetween the Ts T and T The quantity s may be either +1 or 1.

The calculation shows that one obtains at S the output of T T underthese conditions, a difference signal depending upon the accuracy ofpointing of the antenna and at S output of the T T a sum signalindependent of the accuracy of pointing of the antenna.

Given the process of generation of the difference signal, there exists adifference signal for any orientation of the satellite with respect tothe antenna and for any direction of the trajectory thereof.

Such an attenuator may be constituted by a plate of graphited cardboardin the form of a semicircle which one drives in parallel to the smallside of the guide.

Such phase shifters may be constituted by a Teflon plate of thicknessdimensioned to obtain the desired phase shift.

The role of the attenuators is to obtain equality of power in the twooutputs S and S The role of the phase shifters is to furnish aseparation of the modes in the output of the Ts.

As indicated above, the modes T13 and TE may also be extractedseparately by providing two pairs of cavities on the circumference ofthe cylindrical guide at the extremities of two diameters within theguide forming an angle a therebetween for extracting one mode and twoother pairs of cavities on the circumference of the cylindrical guide atthe extremities of two diameters within the guide forming an angle btherebetween. Such an arrangement is illustrated in FIGURES 5a and 5b.The

arrangement of FIGURE 5a for extracting the TE mode corresponds to thearrangement of FIGURE 4 as far as the S circuit is concerned, theelements P and P being short circuits of the sum outputs of the Ts M andM The TE mode is extracted by the arrangement illustrated in FIGURE 51)by means of cavities C C C and C While I have shown and describedseveral embodiments in accordance with the present invention, it isunderstood that the same is not limited thereto but is susceptible ofnumerous changes and modifications as known to a person skilled in theart, and I therefore do not wish to be limited to the details shown anddescribed herein, but intend to cover all such changes and modificationsas are encompassed by the scope of the appended claims.

I claim:

1. Control system for effecting the automatic tracking by a highfrequency receiving antenna of an object omitting a tracking signalcomprising:

a high frequency antenna,

a cylindrical waveguide connected to the output of said antenna,

a first pair of cavities positioned on the circumference of saidwaveguide on either end of a first diameter thereof, said cavities beingcoupled to said waveguide, a second pair of cavities positioned on thecircumference of said waveguide on either end of a second diameterdisposed at an angle to said first diameter, said second pair ofcavities being coupled to said waveguide,

said first and second pair of cavities being proportioned and positionedto be excited by a first mode TE and a second mode T13 said first andsecond pairs of cavities being coupled respectively to four rectangularwaveguides in which are excited the cylindrical modes TE and TE a firstmagic T having two inputs, connected to respective ones of said firstpair of cavities, and having a sum output and a difference output, and asecond magic T having two inputs, connected to respective ones of saidsecond pair of cavities, and having a sum output and a difierenceoutput, the difference outputs of said first and second magic Ts beingcombined to produce a difference signal, a first phase shifter connectedto one of said difference outputs, the sum outputs of said first andsecond magic Ts being combined to produce a sum signal.

2. A control system for efiecting the automatic tracking by a highfrequency receiving antenna of an object emitting a tracking signalcomprising:

a high frequency antenna,

a cylindrical waveguide connected to the output of said antenna,

two first pairs of cavities positioned on the circumference of saidwaveguide on the ends of a first pair of diameters thereof forming anangle a therebetween, said first pairs of cavities being proportionedand positioned to be excited by a mode TE and two second pairs ofcavities positioned on the circumference of said waveguide on the endsof a second pair of diameters thereof forming an angle b therebetween,said second pairs of cavities being proportioned and positioned to beexcited by a mode TE 3. A control system for effecting the automatictracking by a high frequency receiving antenna of an object emitting atracking signal comprising:

a high frequency antenna,

a cylindrical Waveguide connected to the output of said antenna,

means for extracting from said waveguide at least a first pair of energysignals derived from a first mode TE and a second pair of energy signalsderived from a second mode TE and means for combining said first andsecond pairs of energy signals to produce sum and dilference signals forcorrecting the pointing of said antenna.

4. A control system as defined in claim 3 wherein said means forextracting energy from said waveguide includes a plurality of cavitiespositioned on the circumference of said waveguide and being coupled tosaid waveguide to extract energy from said first mode TE and said secondmode TEz 5. A control system as defined in claim 4 wherein said cavitiesare positioned on both ends of a pair of diameters of said waveguide,said diameters being disposed at an angle to one another of 45.

6. A control system as defined in claim 5 wherein said cavities arecoupled respectively to rectangular wave guides in which are excited thecylindrical modes TE and TEg RODNEY D. BENNETT, Primary Examiner.

R. E. BERGER, Assistant Examiner.

